Electric vehicles (electric cars) have been attracting increasing interest in recent years from the viewpoint of prevention of air pollution and reduction of vehicle noise. With these electric cars, so-called regenerative braking can be easily performed. This regenerative braking can be effected by changing over the operation mode of an electric drive motor (hereinafter called the "motor") into an electric power generation mode, whereby rotational energy of the driving wheels is recovered as electric energy through the motor.
Such regenerative braking is generally controlled so that upon depression of a brake pedal or upon release of an accelerator pedal from a depressed position, braking force is produced in association with the depression or release.
When an accelerator pedal is released from a depressed position while a brake pedal is in a non-depressed position, regenerative braking weaker than that exerted when the brake pedal is in a depressed position is performed (this regeneration will be called "moderate regeneration") so that the regenerative brake becomes equivalent to an engine brake in the case of an automotive vehicle driven by an internal combustion engine. If more regenerative braking force than needed is produced at this time, the vehicle speed may, however, drop beyond the driver's desire.
Further, when regenerative braking is performed while driving at a low vehicle speed, such as driving in an urban district, electric power supplied to a motor becomes greater, thereby failing to achieve energy saving.
A technique that makes it possible, by a manual operation, to vary moderate braking force, which is equivalent to an engine brake and applied upon release of an accelerator pedal or a brake pedal in an electric car, has therefore been proposed, for example, in Japanese Patent Application Laid-Open (Kokai) No. HEI 5-122805.
According to the above-mentioned conventional technique of Japanese Patent Application Laid-Open (Kokai) No. HEI 5-122805, however, regenerative braking force is adjusted by a manual operation. While driving on a slope, moderate regeneration may have to be changed frequently in accordance with variations in grade. This imposes an extra burden on the driver.
With the foregoing problem in view, the present invention has as an object to provide of a regenerative braking control system for an electric vehicle, which makes it possible to obtain appropriate regenerative braking force in accordance with conditions of a road, on which the vehicle is running, driver characteristics, and the like without needing frequent manual operations.